# A Status Report on the Phenomenology of Black Holes in Loop Quantum Gravity: Evaporation, Tunneling to White Holes, Dark Matter and Gravitational Waves

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## Abstract

**:**

## 1. Introduction

## 2. Basics of Black Holes in Loop Quantum Gravity

## 3. Modified Hawking Spectrum

#### 3.1. Global Perspective

#### 3.2. Greybody Factors

#### 3.3. Local Perspective

## 4. Bouncing Black Holes

#### 4.1. The Model

#### 4.2. Individual Events and Fast Radio Bursts

#### 4.3. Background

## 5. Dark Matter

## 6. Gravitational Waves

#### 6.1. Spin in Gravitational Wave Observations

#### 6.2. Quasinormal Modes

## 7. Conclusions

- First, the Hawking evaporation spectrum should be modified in its last stages. We have shown that it could not only allow for the observation of a clear signature of LQG effects, but also, in principle, to the discrimination between different LQG models. In particular, holographic models lead to specific features. The value of the Barbero–Immirzi parameter could even by measured.
- Second, attempts to calculate the greybody factors were presented. They should keep a subtle footprint of the polymerization of space and of the existence of a non-vanishing minimum area gap.
- Third, it was emphasized that a local quantum gravity perspective would lead to an observable modification to the Hawking spectrum (line structure), even arbitrarily far away from the Planck mass. This prediction is not washed out by the secondary emission from the BH.
- Fourth, a model with BHs bouncing into white holes with a characteristic time proportional to ${M}^{2}$ was presented and shown to have astrophysical consequences. It can be fine-tuned to explain ether fast radio bursts or the Fermi gamma-ray excess, depending on the values of the parameters. The possible associated background was also studied. A specific redshift dependence allows one to discriminate the model from other possible explanations.
- Fifth, the possibility of having a large amount of dark matter in the form of white holes appearing after quantum gravitational tunneling is presented together with possible weaknesses and future improvements of the model.
- Sixth, observable effects on gravitational wave detections associated with the BHs’ spin distribution expected are presented.
- Seventh, promising prospects for quasinormal modes are outlined.

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Number of BHs that would have to be observed as a function of the relative error on the energy measurement for different confidence levels (the color scale corresponds to the number of standard deviations). Upper plot: discrimination between loop quantum gravity (LQG) and the Hawking spectrum. Lower plot: discrimination between LQG and the Mukhanov–Bekenstein hypothesis [40]. From [38].

**Figure 2.**Spectrum of a holographic black hole for different values of $\gamma $ as a function of $\Delta A$. From [44].

**Figure 3.**$\gamma $ dependence of the integrated spectrum, as function of the energy of the emitted particle, in the holographic model, with a detector energy resolution of 5%. From [44].

**Figure 4.**Emission cross-section for a scalar field with energy $\omega $ for a loop BH of mass M for different values of $\u03f5$. From bottom to top: $\u03f5={10}^{\{-0.3,-0.6,-0.8,-1,-3\}}$. The blue line, corresponding to $\u03f5={10}^{-3}$, is superposed with the cross-section for a Schwarzschild BH. From [56].

**Figure 5.**Emission cross-section for a fermionic field, with energy $\omega $, for a loop BH of mass M. From bottom to top: $\u03f5={10}^{\{-0.3,-0.6,-0.8,-1,-3\}}$. The dashed dark curve corresponds to the Schwarzschild cross-section. From [56].

**Figure 6.**Causal diagram for a bouncing black hole, from [69]. (I) is flat; (II) is Schwarzschild; and (III) is the “quantum gravity” region.

**Figure 7.**Electromagnetic flux emitted by bouncing BHs for a mean mass ${M}_{0}$ of (from right to left) ${M}_{{t}_{H}}$, $10{M}_{{t}_{H}}$, $100{M}_{{t}_{H}}$ and $1000{M}_{{t}_{H}}$, normalized such that the total mass going into primordial black holes (PBHs) is the same. From [81].

**Figure 8.**Measured wavelength, normalized to the rest-frame one, as a function of the redshift. The upper curve is for a conventional astrophysical signal, and the lower one is for bouncing black holes. Reproduced from [82], with the permission of AIP Publishing.

**Figure 9.**Maximum distance at which a single bouncing BH can be observed through its low-energy component, as a function of the k parameter, from [83] (Copyright IOP Publishing. Reproduced with permission. All rights reserved.).

**Figure 10.**Fit to the Fermi excess with bouncing black holes. Reprinted from [82].

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Barrau, A.; Martineau, K.; Moulin, F.
A Status Report on the Phenomenology of Black Holes in Loop Quantum Gravity: Evaporation, Tunneling to White Holes, Dark Matter and Gravitational Waves. *Universe* **2018**, *4*, 102.
https://doi.org/10.3390/universe4100102

**AMA Style**

Barrau A, Martineau K, Moulin F.
A Status Report on the Phenomenology of Black Holes in Loop Quantum Gravity: Evaporation, Tunneling to White Holes, Dark Matter and Gravitational Waves. *Universe*. 2018; 4(10):102.
https://doi.org/10.3390/universe4100102

**Chicago/Turabian Style**

Barrau, Aurélien, Killian Martineau, and Flora Moulin.
2018. "A Status Report on the Phenomenology of Black Holes in Loop Quantum Gravity: Evaporation, Tunneling to White Holes, Dark Matter and Gravitational Waves" *Universe* 4, no. 10: 102.
https://doi.org/10.3390/universe4100102